716 HANDBOOK OF PHOTOGRAPHY 



In this picture, a building which is 88 ft. across will have an image size of only 3^^o in. 

 This question of scale and coverage must be settled in every instance by a study of the 

 purposes for which the picture is intended. Oftentimes, it is not possible to get a 

 satisfactory amount of detail of a large tract all in one picture. In such a case, a 

 series of pictures must be taken, perhaps some from close up and others of a general 

 nature. It should be kept in mind that an overlapping series of oblique pictures can 

 not be mosaiced into a unitary picture. 



After all the details of the flight have been planned, it is the pilot's duty to get the 

 airplane to the proper place and at the proper altitude. Oblique pictures should not 

 be shot with the camera axis pointing at right angles to the airplane because at this 

 angle the speed of the airplane is almost certain to result in blurring. The airplane 

 should be maneuvered so that the camera can be pointed as far toward the tail of the 

 ship as possible without actually photographing the tail. As the pilot approaches the 

 point at which the photograph is to be taken, he will throttle back the ship so as to mini- 

 mize engine vibration, pull the nose of his ship up so as to slow down the speed, and when 

 he gets to the point of exposure which he has probably been approaching at right angles 

 to the direction from which the photograph is to be taken, he should turn the ship 

 slowly away from the object, giving the photographer a chance to shoot toward the 

 tail when the desired position is reached. 



The photographer will find that it takes a considerable amount of practice to handle 

 a full-sized aerial-mapping camera. Owing to vibration imparted from the airplane 

 and angular movements due to not holding the camera steady, a fairly large percentage 

 of blurs appear in the work of beginners. Many oblique photographers support the 

 camera with a sling of rubber shock cord while others employ the practice of holding 

 the camera entirely clear of the airplane structure, pulling it back snugly against their 

 chest at the instant of exposure. 



If the aerial photographer is adept at his work, he will be able to take about three 

 pictures, each time the airplane passes the subject, one slightly before the desired 

 position, one right at it, and one slightly after. With a suitable camera the photog- 

 rapher should be able to take about one picture every 6 sec. 



Flying for Vertical Photographs. — Planning the flight for vertical-map photography 

 is of the utmost importance. First, the best suitable map of the area should 

 be secured. In most of the developed parts of the United States, the maps published 

 by the U. S. Geological Survey are available at stationery stores and afford the best 

 type of information for a flight map. If the flight is of an area of which no suitable 

 maps exist, it is necessary to have a person who knows the boundaries to be 

 mapped fly along in the airplane and point them out or else the corners must be marked 

 on the ground. This may be accomplished by marking the corners with long strips 

 of cheap white cloth, in the shape of an L, the apex being at the corners of the property. 



In most instances, U. S. Geological Survey maps will be available. On this map 

 the boundaries of the property should be indicated by lines drawn in colored pencil. 

 All lines on the flight map should be heavy so as to be seen at a glance bj^ the pilot. 

 After the boundaries are plotted, the direction of flight lines must be determined. If 

 the terrain is flat and if there is no good reason for flying in some other direction, it is 

 customary to fly north and south lines. If, on the other hand, the area is of a long, 

 narrow valley running in some other direction, the most economical flight arrangement 

 is to have the flight lines parallel to the length of the job. 



If the terrain is mountainous, the preparation of the flight map becomes more 

 complicated. In order to secure pictures at the desired scale, the area is frequently 

 zoned according to ground elevation. In other words, an attempt is made to have the 

 airplane fly as nearly as possible at a constant altitude above the various zones of 

 ground elevation. In this case, the flight lines should preferably be parallel to the 



